1. Field of the Invention
This invention relates generally to new and improved devices for detecting electrostatic discharge (ESD) events occurring in electronic components and electronic assemblies and, more particularly, to reusable detectors which can be positioned directly on or closely adjacent to miniature electronic components and printed circuit board assemblies to detect the occurrence, polarity and approximate magnitude of an ESD event. The invention also relates to devices to protect electronic components and circuits from static discharge, in that the invention can be connected to an electrostatic discharge sensitive device so that ESD events above a predetermined magnitude would result in the destruction of the apparatus of the invention, rather than the ESD sensitive device.
2. Description of the Related Art
Static charges nay accumulate on the surfaces of non-grounded conductors and non-conductive surfaces such as most plastics and textiles. These charges will usually remain on the surfaces because there is no path to ground. When a non-grounded conductor comes close to a grounded plane, a spark of high voltage, and potentially destructive low current, will "leap" from a point on the non-grounded conductor to the grounded object, causing an electrostatic discharge (ESD) event. When a charged non-conductor comes close to a conductive object, the charge can be induced onto the conductor, which can then rapidly discharge to other conductors.
Electrostatic discharge occurs in many industrial situations, such as manufacturing and assembly processes, electronic testing processes and the like. For example, in the manufacture of semiconductor devices, electrostatic charges may build up and become discharged during various human or machine handling operations wherein semiconductor wafers are processed, tested and packaged. The amount of electrostatic charge accumulated and discharged during handling of work pieces can be sufficiently high to cause a significant number of component failures, reducing the yield of the various manufacturing, testing and packaging operations and thereby increasing the overall cost of the device.
Static electricity can create a wide variety of problems for electronic manufacturers. An ESD event can cause a rapid electron movement through the microscopic conductive paths within a device and generate a heat spike which can cause damage to the gates or other insulating parts of the electronic device. If the discharge is large enough, a portion of the device will be destroyed and the defect will be found during testing. While high levels of electrostatic discharge will result in the immediate destruction of the device, which can be readily discovered during subsequent testing, low level electrostatic discharge may cause latent damage to the device, which may not be detected during initial testing. This latent damage may later result in reduced performance and/or premature product failure.
At present, efforts have been directed toward prevention of ESD events during manufacturing, since there are few known methods to monitor actual events. Knowing where, how large, and when an event occurs is useful in evaluating ESD induced failures so that appropriate preventive measures can be taken to eliminate the source of the ESD event.
Improvements in the manufacture of semiconductor devices have resulted in devices having vastly increased circuit density, reduced active element size and reduced conductor widths. These improvements have increased the overall performance of the devices, but have simultaneously increased the susceptibility of the devices to damage from electrostatic discharge. As a result, electronic devices are potentially susceptible to damage from discharge events as low as 50 volts. Thus, semiconductive devices in routine manufacture and use today are more susceptible than humans, who normally can not feel an electrostatic discharge of less than approximately 3500 volts.
A variety of instruments have been designed and developed to measure electrostatic phenomena in semiconductor device assembly areas. Some of these devices are connected directly to the circuit boards, while other instruments have antennas or other sensors that detect electromagnetic radiation resulting from an electrostatic discharge. In general, these instruments suffer from one or more disadvantages that limit their acceptance and use in the electronic industry, and similar industries. These disadvantages include being too large or expensive, difficult to monitor in real time, and non-reusable, or some combination of these drawbacks. These prior art instruments also generally fail to provide sufficient information to assist in the detection of devices that are damaged or destroyed, including information leading to the detection and elimination of the incipient environmental causes of the ESD events.
One prior art device used to detect ESD events utilizes a silicon Field Effect Transistor (FET) which is destroyed in the process. This device is monitored by first removing it from the circuit board and then inserting it into an external reader. This device has inherent disadvantages which include its inability to be reused, the requirement that it be removed from the circuit to be tested, its inability to measure polarity, and its limited range of one ESD sensitivity level per device.
Other prior art detection devices are known which utilize a liquid crystal display as an indicator and has a clip lead which can be connected to the particular position of interest, e.g., input to an ESD sensitive device. This particular device has a built-in antenna which senses the ESD event and includes hardware for mounting and protecting the device while in use. This device also has inherent disadvantages which include the large size of the unit, its low operating/storage temperature range, its ability to detect only one transient voltage, and the high cost in manufacturing the unit. Additional disadvantages include incompatibility with automatic insertion equipment and the inability of the device to measure polarity.
What has been needed, and heretofore unavailable, is a reliable, low cost, rugged, miniature, reusable device for accurately and economically detecting the occurrence, polarity and magnitude of electrostatic discharge events, including relatively low level events, which can also provide protection to semiconductive devices from large electrostatic discharges. Such a device should be capable of memory retention, so that the occurrence of an ESD event can be detected anytime after it happens. The present invention satisfies these and other needs.